Numerous electronic devices comprise a multilayered ceramic structure referred to as a "stack" between each ceramic layer of which is sandwiched a patterned electrical conductor. Since each layer provides a substrate for a substantially planar circuit, it is also referred to as a `ceramic circuit substrate`. One or more circuits are typically provided with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, and the like. Electrical interconnection between circuits in successive layers may be provided through "vias" formed by vertical passages filled with a `metal paste`, typically tungsten oxide, molybdenum oxide, or gold, prior to lamination of individual solid but flexible ceramic sheets (also referred to as "tapes") to form the stack, and before the stack is sintered.
A `slip` for a green ceramic sheet is typically prepared by mixing several oxide ceramic particles, with or without glass particles (together referred to as "ceramic powder"), a relatively low molecular weight (mol wt) dispersant, a thermoplastic matrix polymer to reinforce the structure of the green sheet, a plasticizer, and enough solvent to allow the slip to be spread (doctored) onto a moving belt. When the solvent is volatilized, a coherent flexible green sheet is formed. This green sheet is cut into cards of chosen sizes, typically less than 1.5 mm thick, and smaller than 0.5 meter.times.0.5 meter, and fired at a temperature sufficient to drive off the organic components (matrix polymer, plasticizer, dispersant optionally with another steric stabilizer or rheology modifier) and sinter the ceramic particles together into a densified ceramic substrate. At the same time, the metal oxide paste is converted to a sintered dense metal which has the requisite conductivity to provide the desired interconnection.
Since maintaining the conductivity of all circuits is critical to the performance of an electronic device using a stack, especially if the device is to be used over a long period of time in a moisture-containing environment, it will immediately be recognized that the hermeticity of a stack must be maintained.
The problem is that a stack, despite being laminated under pressure, tends to delaminate when it is sintered, for any one or more of a number of different reasons, for example, the rate at which different layers sinter, the different temperatures at which the ceramic sheet and the metal paste are sintered, the difference in thermal expansion coefficients of the ceramic substrate and the metal paste, or the rate at which the polymeric binder is driven off.
It is self-evident that if successive cards in the stack were perfectly hermetically adhered, and the various rates were adjusted so as to provide a compromise of conditions at which the effects of competing stresses were neutralized, no delamination would occur. The problem is to provide as dense a green ceramic sheet as can be cast, which sheet when laminated to form a stack of at least three layers, will not delaminate.
The key to making a high quality stack of high strength, homogeneous sheets, is to have each sheet sufficiently dense, with oxide ceramic particles as nearly perfectly packed as practical, which in turn, depends upon obtaining an excellent dispersion of the particles in the volatile organic solvent(s) used to make a slip from which the tape is formed; and to obtain high density green sheet without vitiating the polymer to particle coupling. Cards cut from the green sheet will then have essentially identical, optimum properties.
One can only obtain maximum green sheet density with maximum sediment density which results in a correspondingly dense cast slip having optimum packing of particles. To obtain optimum packing one must use particles with as narrow a size distribution as possible, allow them to settle slowly out of suspension, and to do so without agglomeration. To prevent agglomeration, the dispersant generates repulsion between adjacent particles. Such repulsion is provided by a mechanism referred to as "charge stabilization". An alternative is to provide "steric stabilization", by which polymers attach to particle surfaces forming "clouds" around the particles. When particles approach each other, the overlap of the polymer cloud provides an osmotic pressure which keeps the particles apart. All the foregoing considerations are known (see "Dispersion of Ceramic Particles in Organic Liquids" by P. D. Calvert, R. R. Lalandham, et al. Matl. Res. Soc. Symp. Proc. Vol 73, pg 579, 1986 Materials Research Society).
However, to provide effective steric stabilization, there must be adequate polymer interaction with the particle; and those portions of the polymer chains distally disposed relative to the particles' surfaces must not interfere with the function of the binder (reference to `binder` typically includes the matrix polymer and a plasticizer which may be present, unless otherwise stated). Since the polymeric binder is necessary to provide a tape, it was evident that it may already provide a degree of steric stabilization which might be beneficial. Therefore it would serve no useful purpose to provide a second polymer which might also provide the same function. Further, the structural interaction of chains of the matrix polymer and such other polymer as might be used for steric stabilization, could not be estimated from a knowledge of the structures of the polymeric binder and other polymer. Nor could it be foreseen what the effect, deleterious or not, of such combination of matrix polymer and other polymer might be on the rheology of the slip to be cast, its morphology, or that of a sintered sheet.
With particular respect to the use of a hydrolyzed siloxane (also referred to as a "prehydrolyzed silane" to emphasize the requirement that the silane be hydrolyzed prior to addition to the dispersion of particles in solvent) which generates a polysiloxanol oligomer (hereafter "oligomer" for brevity), it was not known how strong the affinity of OH groups would be for the oxide ceramic and glass particles used in tape, nor what the interaction of the OH groups would be with the matrix polymer or plasticizer used, even if there was no chemical reaction between the functional groups of the oligomer and those of the matrix polymer and plasticizer. Specifically, it could not be deduced whether the OH groups of the oligomer would be available to the particles, or that they would be available in an amount sufficient to provide effective dispersivity in a slip containing a substantial, though minor amount by weight of binder.
Still further, since a practical tape is made from particles of different oxides, each used in a different range of particle sizes, there is no known method to estimate the dispersive effect of oligomer on the individual particles. For practical reasons, using conventional dispersants, a slip for a card typically contains a major proportion by weight of weight of smaller particles than about 3 .mu.m, there being no difficulty casting a slip of only the large particles. For example, gamma or .alpha.-alumina particles in the range from about 3 .mu.m-6 .mu.m may be used with chromia and glass particles in the range from about 1 .mu.m-4 .mu.m. Using a polysiloxanol oligomer as dispersant, a slip may be made and cast from particles smaller than 3 .mu.m, as small as 0.5 .mu.m.
Because it is difficult to form a desirable dispersion of micron-sized particles with steric stabilization, the emphasis to date, has been on charge stabilization which is provided by a dispersant molecule with a "head" function which attaches itself to the ceramic particle, and a "tail" function responsible for generating repulsion. Siloxanes ("silanes"), carboxylic acids and the like are known effective dispersants, the effectiveness of the former deriving from the organofunctional group on the Si atom. The effectiveness of a dispersant is measured by a sediment volume test in which a ceramic powder, polymeric binder, solvent, and dispersant are thoroughly dispersed by milling, then allowed to settle under gravity in a graduated cylinder. The most effective dispersants result in a sediment density of about 50% of theoretical density (of the ceramic oxide).
Of particular interest is that such densification of particles as may occur with prehydrolyzed tetraalkoxysilanes is unsuitable for the purpose at hand. Oligomerization of a tetraalkoxysilane results in a glassy polysiloxane with no organofunctional group to provide better dispersion. Such a polysiloxane would act as a sintering aid during firing.
When densification occurs with a polysiloxanol derived from a trialkoxysilane, it occurs without disruption of the polymer-to-particle coupling only with certain organic solvents, and when a compatible organofunctional group of the silane (to be hydrolyzed) is chosen. Even with the use of solvents with which such densification does occur, there is no recognized scientific principle to support a conclusion that such densification would be carried over to a suspension of the particles in the presence of a relatively large amount of a polymeric binder, necessarily also present, which binder is soluble in the solvent used.
As earlier stated, after a slip is cast it is dried to form the green ceramic tape from which sheets are cut. Such drying is conducted under elevated temperature to volatilize the solvent quickly. Under controlled drying conditions, a large number of the chains of oligomer not attached to the particles of ceramic powder, are condensed through their OH groups to form a polysiloxane polymer.
Not by any means of minor importance is the ability of a first sheet of cast and dried tape to bond to a second sheet of the tape when the two sheets are overlaid, one upon the other, under pressure. Since typically, a small excess of dispersant is used, whether silane or oligomer, the effect of the excess on the bonding of one sheet to another is critical to the formation of an acceptable stack. It could not be deduced from known scientific principles, how excess polysiloxanol chains would migrate through a cast slip, or why they should, and what the effect of their auto-condensation into polysiloxane chains (when the slip is dried) would be on the adhesion of one dried green sheet to another. Moreover, the greater the excess of oligomer used, the greater the amount of polysiloxane formed, and the higher the rigidity of the dried tape. How such rigidity might affect the processability of the tape could not have been predicted, nor could their proclivity, when sintered, to form a stack having hermetically sealed interfaces between successive sheets.